The exposed catalyst surface area of a supported catalyst is perhaps the most pursued quantity in the area of catalytic research. Dispersion and specific activity derivable from this measurement are essential for characterization of catalytic reactions. This measurement also gives a measure of the efficiency of dispersion of a supported catalyst. Several methods are available for the measurement, such as electron microscopy, X-ray techniques including diffraction and scattering, and gas chemisorption. (as described by R. J. Farrauto in A.I.Ch.E. Symposium Series, 70 (143),9,1974). Of these methods, the gas chemisorption method is perhaps the most accurate and certainly the easiest to implement. Chemisorption has been successful in a number of systems; e.g. H.sub.2 on Pt, H.sub.2 on Ni, CO on Pd, and NO on oxides of Cu, Ni, and Fe. However, several difficulties limit the utility of chemisorption in determining active surface area, the most important being the question of stoichiometry between surface atoms and adsorbed gas. In addition, only a fraction of the exposed surface is covered by chemisorbed gas molecules for some catalysts, which is usually true for catalysts of metal compounds such as oxides or carbonates. These uncertainties along with the difficulties involving the interactions of gas with catalyst in specific systems have prevented the development of a universal method for active surface area measurement using chemisorption.
Physisorption is the physical adsorption of a gas where there are no chemical attraction forces involved in the adsorption. The physisorption of a gas near its liquefaction point has long been used for the determination of total surface area of a solid. In catalysis, it has been used to determine total support area and pore structure. Because of the nonspecific attractive forces of the typical physisorbed gases, such as nitrogen, physisorption historically contributed little to the determination of the catalyst area. A combination of physisorption and chemisorption has been used by Emmett and Brunauer, J.Am. Chem. Soc., 62, 1732(1940), for the determination of the areas of promoters in iron catalysts.
The very specific nature of interactions between surface atoms and chemisorbed gas, which allows determination of the active surface area, is itself the limiting factor for the application of the chemisorption method to catalysts of metal compounds and metals of unknown stoichiometry. The nondiscriminatory nature of physisorption, on the other hand, makes it unsuitable for the active surface area measurement, yet it offers the advantage that the whole surface is covered by gas molecules regardless of the type of surface involved. It has been found that a universal method based on physisorption can be made applicable to any surface for the determination of catalyst surface area provided that two requirements are met. These requirements are: (1) monolayer coverage of catalyst surface by gas molecules, (2) a means by which the physisorption process can be made selective and specific. The first requirement is not difficult to meet since a proper selection of pressure for given adsorbate and temperature ensures monolayer coverage as the well known BET equation (Brunauer, Emmett, and Teller," Adsorption of Gases in Multimolecular Layers", J. Amer. Chem. Soc. 60, 309., 1938) amply demonstrates. The only requirement here is that the catalyst surface be covered by monolayer of gas molecules and not that the coverage be unity. The very fact that the major interaction in multilayer physisorption is between the first layer of gas molecules and surface atoms (or molecules) ensures monolayer coverage at least at a coverage less than unity. The second requirement is much more difficult to meet. Nevertheless, transient desorption behavior, if not steady state, can exhibit some discriminating features provided that the activation energy for desorption is greater than about 3 kcal/mol, preferably 3-7 kcal/mol. If unique but different characteristics of desorption behavior can be found for catalyst and support, these can be utilized for the determination of the fraction of total surface area occupied by the catalyst.
It is, therefore, an object of this invention to provide a novel method and apparatus for measuring the proportion of surface area of a coated substrate that is covered by the coating, more specifically the proportion of the surface area of a supported catalyst that is covered by the catalytic material. It is another object of this invention to provide such a method and apparatus employing physisorbed gas as the medium for measurement of surface area. Other objects will become apparent from the more detailed description of this invention which follows.